5.2逆波兰计算器及中缀表达式转后缀表达式
计算后缀表达式的思路
中缀表达式转后缀表达式
代码实现
package PolandNotation;
import java.util.ArrayList;
import java.util.List;
import java.util.Stack;
public class PolandNotationByStack {
public static void main(String[] args) {
String suffixExpression = "10 6 9 3 + -11 * / * 17 + 5 +";
String infixExpression = "1+((222+31)*4)-5";
List<String> sufList = getListByString(suffixExpression);
// List<String> list = changeInfixExpressionToList(infixExpression);
// List<String> list1 = changeInfixListToSuffixList(list);
// int res = calculate(list1);
int res1 = calculate(sufList);
// System.out.println(res);
System.out.println(res1);
}
public static List<String> getListByString(String poland) {
List<String> list = new ArrayList<>();
String[] split = poland.split(" ");
for(String ele: split) {
list.add(ele);
}
return list;
} // 将后缀字符串转化为列表
public static int calculate(List<String> list) {
Stack<String> stack = new Stack<>();
int num1 = 0;
int num2 = 0;
for(String item: list) {
if (item.matches("[-+]?\d+(?:\.\d+)?")) { // 匹配多位数
stack.push(item);
} else {
num2 = Integer.parseInt(stack.pop());
num1 = Integer.parseInt(stack.pop());
int res = 0;
if (item.equals("+")) {
res = num1 + num2;
} else if(item.equals("-")) {
res = num1 - num2;
} else if(item.equals("*")) {
res = num1 * num2;
} else if(item.equals("/")) {
res = num1 / num2;
} else {
System.out.println("非法字符");
}
stack.push("" + res);
}
}
return Integer.parseInt(stack.pop());
} // 计算后缀表达式
public static List<String> changeInfixExpressionToList(String str) { // 将中缀表达式转化成列表
List<String> ls = new ArrayList<>();
int i = 0;
String mulNumber;
char ch;
do{
if((ch=str.charAt(i)) < 48 || (ch=str.charAt(i)) > 57) {
ls.add("" + ch);
i++;
} else {
mulNumber = "";
while ((i < str.length()) && ((ch=str.charAt(i)) >= 48) && ((ch=str.charAt(i)) <= 57)) {
mulNumber += ch;
i++;
}
ls.add(mulNumber);
}
}while (i < str.length());
return ls;
} // 将中缀字符串转化成中缀列表
public static List<String> changeInfixListToSuffixList(List<String> ls) {
Stack<String> stack = new Stack<>();
List<String> list = new ArrayList<>();
int num1 = 0;
int num2 = 0;
String opera;
for(String str : ls) {
if(str.matches("\d+")) {
list.add(str);
} else if (str.equals("(")) {
stack.push(str);
} else if (str.equals(")")) {
while(!stack.peek().equals("(")) {
list.add(stack.pop());
}
stack.pop();
} else {
while (stack.size() != 0 && operaPriority(str) <= operaPriority(stack.peek())) {
list.add(stack.pop());
}
stack.push(str);
}
}
while (stack.size() != 0) {
list.add(stack.pop());
}
return list;
} // 将中缀列表转化成后缀列表
public static int operaPriority (String str) {
int res;
if (str.equals("+") || str.equals("-")) {
res = 1;
} else if (str.equals("*") || str.equals("/")) {
res = 2;
} else {
res = 0;
}
return res;
} // 返回运算符的优先级
};
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